A bed capable of being lifted in the back bottom member, leg bottom member, etc. has a basic constitution in which the deck is divided into a plurality of regions, say, back region, waist region, leg region and foot region and in which the respective regions are connected bendably with each other or some of them are supported pivotally rotatably on a frame supporting the bottom, so that they can be bent.
However, for example, if the bed is designed to allow the back region and the waist region to be bent by simple pivotal rotation, the mattress supported on the bottom is compressed by the bent back bottom member and waist bottom member, and the bed user such as a patient lying on the mattress is displeased by the pressure felt on the abdomen disadvantageously. To overcome this advantage, various measures have been proposed.
Also the present applicant has proposed various bottom structures which can be curved at a moderate curvature to ensure that the bendable portion between the back region and the waist region or between the waist region and the leg region can suit the bending posture of the human body lying on the bed. For example, the proposals by the present applicant are described in the gazettes of JP, 6-343533, JP, 7-75657, JP, 7-124038 and JP, 9-577. Proposals by other applicants include, for example, the gazette of JP, 10-234523.
Also as described in these previous proposals, one method for achieving the object of obtaining a bottom member capable of being curved at a moderate curvature to suit the bending posture of the human body lying on the bed at the bendable portion between the back region and the waist region or between the waist region and the leg region is to increase the number of bendable points of the bottom member used for the bendable portion.
For example, in the description in said gazette of JP, 10-234523, a back bottom member consists of a liftable portion corresponding to the end side and an extendable and bendable portion corresponding to the waist side.
The extendable and bendable portion consists of three plates which are connected with each other by engagement between connecting shafts and slots, to allow the whole extent of the connected plates to be shortened and extended and to be bent from each other. Of the three connected plates, the plate on one end side is connected bendably with the liftable portion by connecting shafts, and the plate at the other end is connected bendably with the waist bottom member by connecting shafts.
In this constitution, if the liftable portion of the back bottom member is lifted to bend at the connection with the waist bottom member, the three plates constituting the extendable and bendable portion are respectively moved to form a curve. Since the entire region is bent at four points, the region from the liftable portion of the back bottom member to the waist bottom member is curved at a moderate curvature while being extended as a whole, and thus it is intended to avoid that the bed user is displeased by the pressure felt at the abdomen.
In this way, it is effective to increase the number of bending points of the bending portion of a bottom member as one of measures for achieving the object of obtaining a bottom member curved at a moderate curvature. However, the constitution that adjacent plates are engaged with each other using connecting shafts and slots to form a curve in any direction by simple pivotal rotation at the engagement, as adopted in the above extendable and bendable portion, has the following problems.
(a). The extendable and bendable portion has a high degree of freedom in the entire form since the number of bending points is as many as four, but since it can be bent in any direction at the respective bending points, the respective plates do not always move evenly in the transfer from the shortened flat state to the extended curved state or in the transfer reverse to it, and any local load lowers the corresponding plate downward to change the entire form, lowering the stability of the entire form. So, in the extended state, the extendable and bendable portion is not always curved at a proper angle, and it can happen that the bed user feels instability.
(b). The extendable and bendable portion cannot have a desired entire form unless it is manufactured by letting the length of the shortened flat state and the length of the extended curved state are adapted to the actual distances between the liftable portion of the back bottom member and the waist bottom member. For example, if the extended critical length of the extendable and bendable portion is longer than the actual length between the liftable portion of the back bottom member and the waist bottom member, the portion is in a state of hanging down, to fall more than necessary, and if shorter, the portion is raised more than necessary. Furthermore, if the shortened critical length is longer than the actual distance between the liftable portion of the back bottom member and the waist bottom member, the portion is pressed by the lifting portion and the waist bottom member, to be bent at any point.